This invention relates to a distillate fuel derived from the Fischer-Tropsch process, and useful as a diesel fuel. More particularly, this invention relates to a wide cut Fischer-Tropsch derived diesel fuel wherein the distillate boils in a wider range than a conventional diesel fuel while providing favorable low temperature properties and environmentally beneficial effects.
For conventional distillate fuels, e.g., diesel fuels, the final boiling point is determined by a number of factors, including the engines ability to properly combust the tail end of the fuel, density, sulfur and polyaromatic content. These factors increase as end boiling point and T95 (the temperature at which most all the material has boiled off leaving only 5% remaining in the distillation pot) increase and have been shown to have a detrimental effect on emissions. For example, see the Coordinating Research Council (CRC) study on heavy duty diesels in the United States reported in SAE papers 932735, 950250 and 950251, and the European Programme on Emissions, Fuels and Engine Technologies (EPEFE) study on light and heavy duty diesels reported in SAE papers 961069, 961074 and 961075.
The cold filter plugging point (CFPP) is a standard property of oils. IP-309 is an Institute of Petroleum (61 New Cavendish St., London, W.I., England) standard test for cold filter plugging point (CFPP). A similar U.S. standard test is ASTM D6371.
In addition, heavier materials contained in the tail end of the fuel often lead to unfavorable cold flow properties, i.e., cold filter plugging point and cloud point. This is especially true of Fischer-Tropsch derived materials which are highly paraffinic. The heaviest paraffin molecules tend to crystallize as wax particles and precipitate above certain temperatures, resulting in high freeze point or cloud point, or both. Methods for improving cold flow properties of these fuels generally include undercutting the product and hydroisomerizing the distillate. The process of undercutting consists of eliminating the higher molecular weight materials which cause poor low temperature properties by lowering the upper boiling range (cut point) limits for a particular distillate fraction. However, undercutting is unattractive because it reduces the yield of high value marketable product and creates an abundance of off specification materials.
However, emissions measurements on Fischer-Tropsch derived diesel fuels, which have very low sulfur, aromatic and polyaromatic contents resulting in favorable emissions. A report by the Southwest Research Institute (SwRI) entitled xe2x80x9cThe Standing of Fischer-Tropsch Diesel in an Assay of Fuel Performance and Emissionsxe2x80x9d by Jimell Erwin and Thomas W. Ryan, III, NREL (National Renewable Energy Laboratory) Subcontract YZ-2-113215, October 1993, details the advantage of Fischer-Tropsch fuels for lowering emissions when used neat, that is, use of pure Fischer-Tropsch diesel fuels.
Presently, there remains a need to develop an economic distillate fuel, useful as a diesel fuel, which has lower emissions after combustion and allows a greater portion of the distillate to be used as a high value premium product. In particular, emissions of solid particulate matter (PM) and nitrogen oxides (NOx) are an important concern due to current and proposed environmental regulations. In this regard, the ability to incorporate the tail ends of a fuel into a diesel fuel while achieving favorable cold flow properties and lower emissions will provide a distinct economic advantage.
The citations of the several SAE papers referenced herein are:
P. J. Zemroch, P. Schimmering, G. Sado, C. T. Gray and Hans-Martin Burghardt, xe2x80x9cEuropean Programme on Emissions, Fuels and Engine Technologies-Statistical Design and Analysis Techniquesxe2x80x9d, SAE paper 961069.
M. Signer, P. Heinze, R. Mercogliano and J. J. Stein, xe2x80x9cEuropean Programme on Emissions, Fuels and Engine Technologies-Heavy Duty Diesel Studyxe2x80x9d, SAE paper 961074.
D. J. Rickeard, R. Bonetto and M. Signer,xe2x80x9c, xe2x80x9cEuropean Programme on Emissions, Fuels and Engine Technologies-Comparison of Light and Heavy Duty Dieselsxe2x80x9d, SAE paper 961075.
K. B. Spreen, T. L. Ullman and R. L. Mason, xe2x80x9cEffects of Cetane Number, Aromatics and Oxygenates on Emissions from a 1994 Heavy-Duty Diesel Engine with Exhaust Catalystxe2x80x9d, SAE paper 950250.
K. B. Spreen, T. L. Ullman and R. L. Mason, xe2x80x9cEffects of Cetane Number on Emissions from a Prototype 1998 heavy Duty Diesel Enginexe2x80x9d, SAE paper 950251.
Thomas Ryan III and Jimell Erwin, xe2x80x9cDiesel Fuel Composition Effect on Ignition and Emissionsxe2x80x9d, SAE paper 932735.
M. Hublin, P. G. Gadd, D. E. Hall, K. P. Schindler, xe2x80x9cEuropean Programme on Emissions, Fuels and Engine Technologies-Light Duty Diesel Studyxe2x80x9d, SAE paper 961073.
In one embodiment, this invention relates to a wide cut fuel, useful as a diesel fuel, derived from the Fischer-Tropsch process, which reduces emissions and demonstrates favorable cold flow properties. In particular, the fuel comprises a hydrocarbon distillate derived from the Fischer-Tropsch process having a T90 (ASTM D-86) greater than 640xc2x0 F. (338xc2x0 C.) but less than 1000xc2x0 F. (538xc2x0 C.), preferably a T90 greater than 650xc2x0 F. (343xc2x0 C.) but less than 900xc2x0 F. (482xc2x0 C.), more preferably a T90 greater than 660xc2x0 F. (349xc2x0 C.) but less than 800xc2x0 F. (427xc2x0 C.), even more preferably a T90 greater than 660xc2x0 F. (349xc2x0 C.) but less than 700xc2x0 F. (371xc2x0 C.), and has a cloud point (ASTM D-2500-98a) and cold filter plugging point (CFPP) (IP-309) of less than 5xc2x0 C., preferably less than xe2x88x925xc2x0 C., more preferably less than xe2x88x9215xc2x0 C., still more preferably less than xe2x88x9230xc2x0 C. wherein the fuel contains;
Preferably, the fuel of this invention is produced by separating a wax containing Fischer-Tropsch derived product into a 300xc2x0 F.+ distillate fraction which is further upgraded via hydroisomerization and selective catalytic dewaxing. In particular, a 300xc2x0 F.+ (149xc2x0 C.+) fraction derived from the Fischer-Tropsch process is passed into a first reaction zone, of two sequential isomerization reaction zones in a single reaction stage, the first reaction zone comprising a first catalyst containing a suitable hydroisomerization catalyst, to form a first zone effluent. At least a portion of the liquid product from the first zone effluent, preferably the entire liquid product from the first zone effluent, is passed into a second reaction zone, comprising a second catalyst having a catalytic dewaxing functionality, to form a second zone effluent. In the alternative, the second reaction zone may contain a mixture or composite comprising both catalytic dewaxing and hydroisomerization catalysts. The first and second zones may be in the same or separate reaction vessels and preferably both zones are contained in the same reaction vessel. Further, the first and/or second reaction zone may comprise one or more catalyst beds. The second zone effluent comprises an isomerized hydrocarbon product and can be fractionated into desired liquid product fractions, e.g., a 320-700xc2x0 F. boiling fraction.
By 300xc2x0 F.+ fraction is meant the fraction of the hydrocarbons synthesized by the Fischer-Tropsch process and boiling above a nominal 300xc2x0 F. boiling point. At least a portion of the product of the second reaction zone is recovered to produce a middle distillate boiling in the diesel fuel range, i.e., a 320-700xc2x0 F. boiling fraction. Preferably, the process is conducted in the absence of intermediate hydrotreating, and produces products with excellent cold flow characteristics, i.e., cloud and freeze point, superior smoke point and better than expected emissions characteristics.
A T90 for a typical diesel fuel is approximately 540xc2x0 F.-640xc2x0 F. (282xc2x0 C.-338xc2x0 C.), see ASTM D-975-98b. However, smoke levels, emissions and unfavorable cold flow properties generally increase with boiling temperature. See SAE 961073 and 961069. The fuel of this invention comprises a wide cut fuel which includes high end boiling fractions, but still demonstrates favorable cold flow properties while reducing emissions. In addition, the fuel of this invention reduces smoke levels during acceleration.